With OFDM systems, such as those used for 802.11 protocol transmissions, pilot tones are used to assist with transmission and reception. With MIMO systems, or multi-transmit systems, additional complexities arise. MIMO systems can provide transmit diversity gains. With simple wireless transmitters using single transmit spatial streams on single transmit antennas, such benefits are not obtained and even with some pilot tone schemes, some of the benefits might not be obtained.
In a MIMO system, pilot tones might be transmitted using multiple transmit spatial streams by cycling through spatial streams, but then spectral flatness becomes an issue and pilot tracking might fail if there is fading on the spatial stream that happens to be carrying the pilot tone at the time.